White-tailed deer are the most abundant large mammal in North America with a range extending from Canada to South America. The US population alone is estimated to number 30 million animals. They are a social species that live in family groups of two to 12 individuals that can thrive in a range of habitats, including urban parks and woodland.
These aspects of their ecology and behaviour have made them a species of particular concern when it comes to the spread of diseases, including bovine tuberculosis and chronic wasting disease. These pathogens have already led to considerable effects on the health of wild and domestic animal populations around the globe.
The findings from these latest studies have raised concerns that white-tailed deer could be a reservoir of SARS-CoV-2. Not only could this readily infect large numbers of animals, but also, more worryingly, it could spill back to humans.
This type of infection cycle was documented in workers on infected mink farms, which ultimately led to the Danish government euthanising their entire captive population of 17 million animals. It is important to underline that there is currently no evidence of SARS-CoV-2 transmission from white-tailed deer to humans. Initial experimental work has also suggests that infected deer tend not to have symptoms. Still, disease transmission in wildlife populations has considerable implications for human and animal health.
Potential source of new variants
There is the possibility that viral mutation in a reservoir host, such as white-tailed deer, could lead to new variants of the disease. These variants may lead to greater infection rates, increased virulence (severity of symptoms) and prove more effective at evading the human immune system. Likewise, any reinfection from wildlife reservoirs could also complicate our long-term efforts to fight and suppress the disease.
Influenza, which jumps readily between birds, humans and other mammals (particularly pigs), presented similar problems. These multiple reservoirs of disease can lead to new strains emerging that humans have lower immunity against, as was the case with swine flu in 2009.
It is important to note that there are limitations to these studies, both in terms of the methods used and the limited geographical range of investigation. The most recent and unpublished study used the latest genetic approaches to reliably detect SARS-CoV-2 in tissue samples but focused only on deer in Iowa. Whereas the antibody tests in the first study were conducted across four states but only show that the animal has been exposed to the virus. Yet the combined findings have highlighted that transmission of SARS-CoV-2 is likely to be widespread in white-tailed deer.
There is a great deal that we still need to learn about the developing situation with COVID and deer. The most important topics to focus on include understanding how the virus is being transmitted from humans to deer and determining the risk of spillover back into the human population. Research is urgently needed to assess the risk that this potential reservoir of SARS-CoV-2 presents to humans, as well as the possible spread of the virus to other wildlife species that deer interact with, such as predators and scavengers.
